This invention relates generally to semiconductor devices, and, more specifically, to a semiconductor device having a very steep retrograde distribution of a rapidly diffusing species in a near-surface layer of a substantially perfect base crystal.
The proper operation of miniature electronic devices often depends significantly upon the ability to fabricate specialized forms of dopant layers within crystals. As an example, the fabrication of submicrometer CMOS integrated circuits using either bulk silicon or silicon-on-sapphire (SOS) substrates requires the fabrication of properly scaled submicrometer n- and p-channel MOS Field Effect Transistors (MOSFETs) with ultralow source and drain resistances, and minimal short channel behavior. The fabrication of such MOSFETs in turn necessitates the fabrication of dopant layers with high junction gradients, shallow junction depths, and ultralow sheet resistances.
The formation of P.+-. and N.+-. doped source and drain regions requires the formation of shallow, hyperabrupt distributions of ions such as boron, phosphorus, or arsenic in substantially perfect silicon crystals. In particular, the junctions of doped regions should lie within less than about 0.2 micrometers of the surface of the silicon crystal, and should include a retrograde junction having a gradient on the order of the solubility of the dopant species over a small distance, preferably about 0.01 micrometers. Further, when fabrication of the device is complete, the sheet resistance of the doped junction layer should be less than about 100 ohms per square, in turn requiring that the silicon base crystal be substantially free of defects and that the dopants be fully activated.
The fabrication of shallow junctions having steep concentration gradients, together with the requirement of low sheet resistance, poses difficult fabrication problems, so that such junctions have heretofore been impossible to prepare. In typical prior approaches, the dopant species was implanted into the surface of the silicon crystal under known and controlled conditions in an attempt to achieve the desired steep retrograde distribution, but the implanted crystal was then heated to elevated temperature to activate the implanted species and also to anneal the damage introduced by the implantation procedure, inasmuch as such damage increases the sheet resistance of the final device. This annealing procedure causes diffusion of the implanted species, which acts to reduce the steepness of the concentration gradient at the junction, and to cause migration of the junction to deeper depths within the crystal. Moreover, the implantation procedure and the annealing step can result in channelling of the implanted species, so that "tails" of high concentrations of the implanted species can be found along particular crystallographic directions in the base crystal following these procedures. Channelling effects tend to destroy the uniformity of the junction, also reducing the gradient and migrating the junction to deeper depths within the crystal.
Thus, in prior attempts to produce the necessary shallow junctions with steep concentration gradients, in a crystal having a low sheet resistance, the dopant ions were first implanted and then the crystal was annealed at a temperature of about 900.degree.-950.degree. C. for about 30-60 minutes. Athough the implantation procedure produces the desired dopant distribution to a first approximation, the distribution is imperfect because of the channelling tails and broadening due to the subsequent annealing procedure, as the junction migrates to deeper depths within the crystal and the sharpness of the concentration gradient at the junction is reduced. The resulting deep junctions can cause detrimental short channel behavior in submicrometer n- p-channel MOSFETs. The annealing procedure which causes the modification of the junction cannot be omitted, because the annealing process is required to activate the implanted species and to reduce the defect density in the base crystal for a sufficiently low sheet resistivity. For the case of SOS devices, the problem is further aggravated by the high defect density in the silicon film, which reduces dopant activation and enhances lateral diffusion under the gate of p-channel MOSFETs.
Thus, the fabricator of devices must reduce the defect density of the underlying base crystal to achieve the necessary low resistivities, but the conventional implanting and annealing procedure results in channelling of the implanted species and also broadening and deepening of the implanted junction. If the implanted species did not diffuse within the base crystal, the annealing treatment would not adversely modify the junction characteristics. Unfortunately, however, the dopant species of principal interest such as boron, phosphorus, and arsenic do diffuse rapidly within silicon base crystals at typical annealing and activation temperatures, and it has not been possible to prepare the desired junctions by prior procedures.
Thus, there has been proposed no fabrication procedure for producing hyperabrupt, shallow junctions of such dopants in base crystals, to yield a device having a low sheet resistivity. Accordingly, there exists a need for such a fabrication procedure. The process should allow the preparation of shallow, steep retrograde junctions of rapidly diffusing dopant species in base crystals, with the desired junction preserved through a treatment to reduce the defect concentration of the base crystal so that the sheet resistivity of the completed device is low. The fabrication procedure should be compatible with existing technology for producing microcircuits, and in particular should be compatible with further fabrication steps to add other components onto the chip. The present invention fulfills this need, and further provides related advantages.